Manipulating the Transition Dipole Moment of CsPbBr3 Perovskite Nanocrystals for Superior Optical Properties

Symmetry-Broken Electronic State of CsPbBr3 Cubic Perovskite Nanocrystals

The development of densely packed, self-assembled perovskite nanocrystals (PeNCs) with a favorable transition dipole moment (TDM) orientation is crucial for their application in solution-processable electronic devices. In this study, we fabricated anisotropic CsPbBr3 PeNCs with a symmetry-broken electronic state on quartz substrates modified by 3-aminopropyltrimethoxysilane (APS). Densely packed and self-assembled monolayers of cubic PeNCs were formed on the substrates by using a dip coating technique. The angle-dependent absorption and photoluminescence (PL) spectra confirmed that the PeNC monolayer on the APS-treated substrate exhibited anisotropic electronic states in the in-plane and out-of-plane directions of the substrate. In contrast, when the quartz substrate was modified with the long alkyl silane coupling agent, octadecyltrimethoxysilane, the absorption and PL spectra exhibited no angular dependence, indicating the absence of anisotropy. Experimental and simulated results confirmed the presence of vertical TDMs in the densely packed PeNCs on the APS-treated substrate, which could be attributed to the effect of the amino groups of the APS on the facet of the cubic PeNCs facing the quartz substrate. Hence, surface chemical modifications of the substrate can aid in the precise control of the symmetry of the electronic states and TDM orientation in cubic PeNCs. These findings can promote the development of densely packed, high-coverage PeNC films with a controllable TDM orientation for applications in electronic devices such as solar cells, sensors, and light-emitting diodes.

Low-threshold cavity-enhanced superfluorescence in polyhedral quantum dot superparticles

Due to the unique and excellent optical performance and promising prospect for various photonics applications, cavity-enhanced superfluorescence (CESF) in perovskite quantum dot assembled superstructures has garnered wide attention. However, the stringent requirements and high threshold for achieving CESF limit its further development and application. The high threshold of CESF in quantum dot superstructures is mainly attributed to the low radiation recombination rate of the quantum dot and the unsatisfactory light field limiting the ability of the assembled superstructures originating from low controllability of self-assembly. Herein, we propose a strategy to reduce the threshold of CESF in quantum dot superstructure microcavities from two aspects: facet engineering optimization of quantum dot blocks and controllability improvement of the assembly method. We introduce dodecahedral quantum dots with lower nonradiative recombination, substituting frequently used cubic quantum dots as assembly blocks. Besides, we adopt the micro-emulsion droplet assembly method to obtain spherical perovskite quantum dot superparticles with high packing factors and orderly internal arrangements, which are more controllable and efficient than the conventional solvent-drying methods. Based on the dodecahedral quantum dot superparticles, we realized low-threshold CESF (Pth = 15.6 μJ cm-2). Our work provides a practical and scalable avenue for realizing low threshold CESF in quantum dot assembled superstructure systems.

Origin of the low-energy tail in the photoluminescence spectrum of CsPbBr3 nanoplatelets: a femtosecond transient absorption spectroscopic study

CsPbBr3 nanoplatelets (NPLs), as some of the two-dimensional lead halide perovskites, have been intensively investigated due to their outstanding photophysical and photoelectric properties. However, there remain unclear fundamental issues on their carrier kinetics and the low-energy tail in their photoluminescence (PL) spectrum. In this paper, we synthesized CsPbBr3 NPLs with five [PbBr6]4- monolayers and performed comprehensive studies by using steady-state absorption, PL, and femtosecond transient absorption (fs-TA) spectroscopic measurements. We determined both the biexciton Auger recombination time (7 ± 2 ps) and trapped exciton lifetime (110 ± 15 ps) of the five monolayer CsPbBr3 NPLs. We also investigated the origin of the low-energy tail emission in their PL spectrum. More importantly, we found that a negative ΔA feature in the energy range of 2.45-2.55 eV appears in their fs-TA spectrum at 2, 4 and 10 ps delay times, which could help them act as a laser gain medium. The low-energy tail emission in their PL spectrum overlaps well with the negative ΔA feature in the energy range of 2.45-2.55 eV in their fs-TA spectrum at 2, 4 and 10 ps delay times.

Self-assembly of perovskite nanoplates in colloidal suspensions

In recent years, perovskite nanocrystal superlattices have been reported with collective optical phenomena, offering a promising platform for both fundamental science studies and device engineering. In this same avenue, superlattices of perovskite nanoplates can be easily prepared on different substrates, and they too present an ensemble optical response. However, the self-assembly and optical properties of these aggregates in solvents have not been reported to date. Here, we report on the conditions for this self-assembly to occur and show a simple strategy to induce the formation of these nanoplate stacks in suspension in different organic solvents. We combined wide- and small-angle X-ray scattering and scanning transmission electron microscopy to evaluate CsPbBr3 and CsPbI3 perovskite nanoplates with different thickness distributions. We observed the formation of these stacks by changing the concentration of nanoplates and the viscosity of the colloidal suspensions, without the need for antisolvent addition. We found that, in hexane, the concentration for the formation of the stacks is rather high and approximately 80 mg mL-1. In contrast, in decane, dodecane, and hexadecane, we observe a much easier self-assembly of the nanoplates, presenting a clear correlation between the degree of aggregation and viscosity. We, then, discuss the impact of the self-assembly of perovskite nanoplates on Förster resonant energy transfer. Our predictions suggest an energy transfer efficiency higher than 50% for all the donor-acceptor systems evaluated. In particular, we demonstrate how the aggregation of these particles in hexadecane induces FRET for CsPbBr3 nanowires. For the n = 2 nanowires (donor) to the n = 3 nanowires (acceptor), the FRET rate was found to be 4.1 ns-1, with an efficiency of 56%, in agreement with our own predictions.

Controlled Growth of Perovskite Nanocrystals on Nanotubes via a Nanoseeding Intermediate Stage: Toward Novel Optoelectronic Applications

CsPbBr3 perovskite nanocrystals (CNCs) were densely anchored on multiwalled carbon nanotubes (MWNTs) via a nanoseeding intermediate stage, in which lead-based nuclei are formed on the nanotube surface. After the formation of the intermediate, a cesium precursor was added to promote the growth of CNCs from the surface nuclei and to thereby obtain CNC-decorated MWNT nanohybrids (CMNHs). The morphology and properties of the CMNHs were determined by the reaction temperature employed during their synthesis. Importantly, the use of MWNTs promoted the formation of larger CNCs that emitted intense green light and modified the electronic structure and bandgap energy of the CNCs. Consequently, the CMNHs could function as optoelectronic transducers and exhibit a "turn-on" photocurrent response when exposed to UV light of narrow specific-range wavelengths. In a novel approach for preventing counterfeit products, the CMNHs were used as a light-emitting black ink to create quick-response codes with fake pixels.

Synthesis of Zwitterionic CsPbBr3 Nanocrystals with Controlled Anisotropy using Surface-Selective Ligand Pairs

Mechanistic studies of the morphology of lead halide perovskite nanocrystals (LHP-NCs) are hampered by a lack of generalizable suitable synthetic strategies and ligand systems. Here, the synthesis of zwitterionic CsPbBr3 NCs is presented with controlled anisotropy using a proposed "surface-selective ligand pairs" strategy. Such a strategy provides a platform to systematically study the binding affinity of capping ligand pairs and the resulting LHP morphologies. By using zwitterionic ligands (ZwL) with varying structures, majority ZwL-capped LHP NCs with controlled morphology are obtained, including anisotropic nanoplatelets and nanorods, for the first time. Combining experiments with density functional theory calculations, factors that govern the ligand binding on the different surface facets of LHP-NCs are revealed, including the steric bulkiness of the ligand, the number of binding sites, and the charge distance between binding moieties. This study provides guidance for the further exploration of anisotropic LHP-NCs.

Light management for perovskite light-emitting diodes

Perovskite light-emitting diodes (LEDs) have reached external quantum efficiencies of over 20% for various colours, showing great potential for display and lighting applications. Despite the internal quantum efficiencies of the best-performing devices already approaching unity, around 80% of the internally generated photons are trapped in the devices and lose energy through a variety of lossy channels. Significant opportunities for improving efficiency and maximizing photon extraction lie in the effective management of light. In this Review we analyse light management strategies based on the intrinsic optical properties of the perovskite materials and the extrinsic properties related to device structures. These approaches should allow the external quantum efficiencies of perovskite LEDs to substantially exceed the conventional limits of planar organic LED devices. By revisiting lessons learned from organic LEDs and perovskite solar cells, we highlight possible directions of future research towards perovskite LEDs with ultrahigh efficiencies.

Back focal plane imaging for light emission from a tunneling junction in a low-temperature ultrahigh-vacuum scanning tunneling microscope

We report the design and realization of the back focal plane (BFP) imaging for the light emission from a tunnel junction in a low-temperature ultrahigh-vacuum (UHV) scanning tunneling microscope (STM). To achieve the BFP imaging in a UHV environment, a compact "all-in-one" sample holder is designed and fabricated, which allows us to integrate the sample substrate with the photon collection units that include a hemisphere solid immersion lens and an aspherical collecting lens. Such a specially designed holder enables the characterization of light emission both within and beyond the critical angle and also facilitates the optical alignment inside a UHV chamber. To test the performance of the BFP imaging system, we first measure the photoluminescence from dye-doped polystyrene beads on a thin Ag film. A double-ring pattern is observed in the BFP image, arising from two kinds of emission channels: strong surface plasmon coupled emissions around the surface plasmon resonance angle and weak transmitted fluorescence maximized at the critical angle, respectively. Such an observation also helps to determine the emission angle for each image pixel in the BFP image and, more importantly, proves the feasibility of our BFP imaging system. Furthermore, as a proof-of-principle experiment, electrically driven plasmon emissions are used to demonstrate the capability of the constructed BFP imaging system for STM induced electroluminescence measurements. A single-ring pattern is obtained in the BFP image, which reveals the generation and detection of the leakage radiation from the surface plasmon propagating on the Ag surface. Further analyses of the BFP image provide valuable information on the emission angle of the leakage radiation, the orientation of the radiating dipole, and the plasmon wavevector. The UHV-BFP imaging technique demonstrated here opens new routes for future studies on the angular distributed emission and dipole orientation of individual quantum emitters in UHV.

YCl3-Substituted CsPbI3 Perovskite Nanorods for Efficient Red-Light-Emitting Diodes

Cesium lead iodide (CsPbI₃) perovskite nanocrystals (NCs) are a promising material for red-light-emitting diodes (LEDs) due to their excellent color purity and high luminous efficiency. However, small-sized CsPbI₃ colloidal NCs, such as nanocubes, used in LEDs suffer from confinement effects, negatively impacting their photoluminescence quantum yield (PLQY) and overall efficiency. Here, we introduced YCl₃ into the CsPbI₃ perovskite, which formed anisotropic, one-dimensional (1D) nanorods. This was achieved by taking advantage of the difference in bond energies among iodide and chloride ions, which caused YCl₃ to promote the anisotropic growth of CsPbI₃ NCs. The addition of YCl₃ significantly improved the PLQY by passivating nonradiative recombination rates. The resulting YCl₃-substituted CsPbI₃ nanorods were applied to the emissive layer in LEDs, and we achieved an external quantum efficiency of ~3.16%, which is 1.86-fold higher than the pristine CsPbI₃ NCs (1.69%) based LED. Notably, the ratio of horizontal transition dipole moments (TDMs) in the anisotropic YCl₃:CsPbI₃ nanorods was found to be 75%, which is higher than the isotropically-oriented TDMs in CsPbI₃ nanocrystals (67%). This increased the TDM ratio and led to higher light outcoupling efficiency in nanorod-based LEDs. Overall, the results suggest that YCl₃-substituted CsPbI₃ nanorods could be promising for achieving high-performance perovskite LEDs.

Confinement-Tunable Transition Dipole Moment Orientation in Perovskite Nanoplatelet Solids and Binary Blends

Tuning the transition dipole moment (TDM) orientation in low-dimensional semiconductors is of fundamental and practical interest, as it enables high-efficiency nanophotonics and light-emitting diodes. However, despite recent progress in nanomaterials physics and chemistry, material systems that allow continuous tuning of the TDM orientation remain rare. Here, combining k-space photoluminescence spectroscopy and multiscale modeling, we demonstrate that the TDM orientation in lead halide perovskite (LHP) nanoplatelet (NPL) solids is largely confinement-tunable through the NPL geometry that regulates the anisotropy of Bloch states, dielectric confinement, and exciton fine structure. We further quantified the role of uniaxial ordering during NPL assembly in modifying the macroscopic emission directionality of thin films, which is especially important in actual optoelectronic devices. Our theoretical framework successfully corroborates the previous prediction of exciton bright level order reversal with experimental evidence of a counterintuitive reduction of in-plane dipole ratio in ultrathin (one- and two-monolayer-thick) NPLs, even at room temperature. More interestingly, the NPLs retain their TDM orientation in binary blends irrespective of interparticle energy transfer, owing to the phase segregation and NPL-NPL decoupling, enabling the design of films whose fluorescence exhibits an intrinsic angle-dependent color gradient.

One-Dimensional Highly-Confined CsPbBr3 Nanorods with Enhanced Stability: Synthesis and Spectroscopy

One-dimensional (1D) colloidal lead halide perovskites (LHPs) have potential as quantum emitters. Their study, however, has been hampered by their previous instability, leaving a gap in our understanding of structure-property relationships in colloidal LHPs with anisotropic shapes. Here, we synthesize stable, highly-confined 1D CsPbBr₃ nanorods (NRs) and demonstrate their structural details and photoluminescence (PL) properties at both the ensemble and single particle levels. Using amino-terminated copolymers, we are able to stabilize and characterize 1D CsPbBr₃ NRs utilizing transmission electron microscopy (TEM) and small angle scattering (SAS). Scanning transmission electron microscopy reveals that these NRs possess structural defects, including twists and inhomogeneity. Solution-phase photon correlation spectroscopy shows low biexciton-to-exciton quantum yield ratios (QY_BX/QY_X) and broad spectral line widths dominated by homogeneous broadening.

Superradiance and Exciton Delocalization in Perovskite Quantum Dot Superlattices

Achieving superradiance in solids is challenging due to fast dephasing processes from inherent disorder and thermal fluctuations. Perovskite quantum dots (QDs) are an exciting class of exciton emitters with large oscillator strength and high quantum efficiency, making them promising for solid-state superradiance. However, a thorough understanding of the competition between coherence and dephasing from phonon scattering and energetic disorder is currently unavailable. Here, we present an investigation of exciton coherence in perovskite QD solids using temperature-dependent photoluminescence line width and lifetime measurements. Our results demonstrate that excitons are coherently delocalized over 3 QDs at 11 K in superlattices leading to superradiant emission. Scattering from optical phonons leads to the loss of coherence and exciton localization to a single QD at temperatures above 100 K. At low temperatures, static disorder and defects limit exciton coherence. These results highlight the promise and challenge in achieving coherence in perovskite QD solids.

Pressure-Induced Indirect-Direct Bandgap Transition of CsPbBr3 Single Crystal and Its Effect on Photoluminescence Quantum Yield

Despite extensive study, the bandgap characteristics of lead halide perovskites are not well understood. Usually, these materials are considered as direct bandgap semiconductors, while their photoluminescence quantum yield (PLQY) is very low in the solid state or single crystal (SC) state. Some researchers have noted a weak indirect bandgap below the direct bandgap transition in these perovskites. Herein, application of pressure to a CsPbBr3 SC and first-principles calculations reveal that the nature of the bandgap becomes more direct at a relatively low pressure due to decreased dynamic Rashba splitting. This effect results in a dramatic PLQY improvement, improved more than 90 times, which overturns the traditional concept that the PLQY of lead halide perovskite SC cannot exceed 10%. Application of higher pressure transformed the CsPbBr3 SC into a pure indirect bandgap phase, which can be maintained at near-ambient pressure. It is thus proved that lead halide perovskites can induce a phase transition between direct and indirect bandgaps. In addition, distinct piezochromism is observed for a perovskite SC for the first time. This work provides a novel framework to understand the optoelectronic properties of these important materials.

Lasing Action from Quasi-Propagating Modes

Band edges at the high symmetry points in reciprocal space of periodic structures hold special interest in materials engineering for their high density of states. In optical metamaterials, standing waves found at these points have facilitated lasing, bound-states-in-the-continuum, and Bose-Einstein condensation. However, because high symmetry points by definition are localized, properties associated with them are limited to specific energies and wavevectors. Conversely, quasi-propagating modes along the high symmetry directions are predicted to enable similar phenomena over a continuum of energies and wavevectors. Here, quasi-propagating modes in 2D nanoparticle lattices are shown to support lasing action over a continuous range of wavelengths and symmetry-determined directions from a single device. Using lead halide perovskite nanocrystal films as gain materials, lasing is achieved from waveguide-surface lattice resonance (W-SLR) modes that can be decomposed into propagating waves along high symmetry directions, and standing waves in the orthogonal direction that provide optical feedback. The characteristics of the lasing beams are analyzed using an analytical 3D model that describes diffracted light in 2D lattices. Demonstrations of lasing across different wavelengths and lattice designs highlight how quasi-propagating modes offer possibilities to engineer chromatic multibeam emission important in hyperspectral 3D sensing, high-bandwidth Li-Fi communication, and laser projection displays.

Quantum Efficiency Enhancement of Lead-Halide Perovskite Nanocrystal LEDs by Organic Lithium Salt Treatment

Surface-defect passivation is key to achieving a high photoluminescence quantum yield in lead halide perovskite nanocrystals. However, in perovskite light-emitting diodes, these surface ligands also have to enable balanced charge injection into the nanocrystals to yield high efficiency and operational lifetime. In this respect, alkaline halides have been reported to passivate surface trap states and increase the overall stability of perovskite light emitters. On the one side, the incorporation of alkaline ions into the lead halide perovskite crystal structure is considered to counterbalance cation vacancies, whereas on the other side, the excess halides are believed to stabilize the colloids. Here, we report an organic lithium salt, viz. LiTFSI, as a halide-free surface passivation on perovskite nanocrystals. We show that treatment with LiTFSI has multiple beneficial effects on lead halide perovskite nanocrystals and LEDs derived from them. We obtain a higher photoluminescence quantum yield and a longer exciton lifetime and a radiation pattern that is more favorable for light outcoupling. The ligand-induced dipoles on the nanocrystal surface shift their energy levels toward a lower hole-injection barrier. Overall, these effects add up to a 4- to 7-fold boost of the external quantum efficiency in proof-of-concept LED structures, depending on the color of the used lead halide perovskite nanocrystal emitters.

Anisotropic nanocrystal superlattices overcoming intrinsic light outcoupling efficiency limit in perovskite quantum dot light-emitting diodes

Quantum dot (QD) light-emitting diodes (LEDs) are emerging as one of the most promising candidates for next-generation displays. However, their intrinsic light outcoupling efficiency remains considerably lower than the organic counterpart, because it is not yet possible to control the transition-dipole-moment (TDM) orientation in QD solids at device level. Here, using the colloidal lead halide perovskite anisotropic nanocrystals (ANCs) as a model system, we report a directed self-assembly approach to form the anisotropic nanocrystal superlattices (ANSLs). Emission polarization in individual ANCs rescales the radiation from horizontal and vertical transition dipoles, effectively resulting in preferentially horizontal TDM orientation. Based on the emissive thin films comprised of ANSLs, we demonstrate an enhanced ratio of horizontal dipole up to 0.75, enhancing the theoretical light outcoupling efficiency of greater than 30%. Our optimized single-junction QD LEDs showed peak external quantum efficiency of up to 24.96%, comparable to state-of-the-art organic LEDs.

Controlling the transition-dipole-moment orientation in quantum dot solids at device level has not been achieved before. Here, the authors demonstrated intrinsic light out-coupling enhancement approach to boost the external quantum efficiency up to 25% by using the colloidal lead halide perovskite anisotropic nanocrystals.

Colloidal Metal-Halide Perovskite Nanoplatelets: Thickness-Controlled Synthesis, Properties, and Application in Light-Emitting Diodes

Colloidal metal-halide perovskite nanocrystals (MHP NCs) are gaining significant attention for a wide range of optoelectronics applications owing to their exciting properties, such as defect tolerance, near-unity photoluminescence quantum yield, and tunable emission across the entire visible wavelength range. Although the optical properties of MHP NCs are easily tunable through their halide composition, they suffer from light-induced halide phase segregation that limits their use in devices. However, MHPs can be synthesized in the form of colloidal nanoplatelets (NPls) with monolayer (ML)-level thickness control, exhibiting strong quantum confinement effects, and thus enabling tunable emission across the entire visible wavelength range by controlling the thickness of bromide or iodide-based lead-halide perovskite NPls. In addition, the NPls exhibit narrow emission peaks, have high exciton binding energies, and a higher fraction of radiative recombination compared to their bulk counterparts, making them ideal candidates for applications in light-emitting diodes (LEDs). This review discusses the state-of-the-art in colloidal MHP NPls: synthetic routes, thickness-controlled synthesis of both organic-inorganic hybrid and all-inorganic MHP NPls, their linear and nonlinear optical properties (including charge-carrier dynamics), and their performance in LEDs. Furthermore, the challenges associated with their thickness-controlled synthesis, environmental and thermal stability, and their application in making efficient LEDs are discussed.

Structure and Surface Passivation of Ultrathin Cesium Lead Halide Nanoplatelets Revealed by Multilayer Diffraction

The research on two-dimensional colloidal semiconductors has received a boost from the emergence of ultrathin lead halide perovskite nanoplatelets. While the optical properties of these materials have been widely investigated, their accurate structural and compositional characterization is still challenging. Here, we exploited the natural tendency of the platelets to stack into highly ordered films, which can be treated as single crystals made of alternating layers of organic ligands and inorganic nanoplatelets, to investigate their structure by multilayer diffraction. Using X-ray diffraction alone, this method allowed us to reveal the structure of ∼12 Å thick Cs-Pb-Br perovskite and ∼25 Å thick Cs-Pb-I-Cl Ruddlesden-Popper nanoplatelets by precisely measuring their thickness, stoichiometry, surface passivation type and coverage, as well as deviations from the crystal structures of the corresponding bulk materials. It is noteworthy that a single, readily available experimental technique, coupled with proper modeling, provides access to such detailed structural and compositional information.

Power-Dependent Photoluminescence Efficiency in Manganese-Doped 2D Hybrid Perovskite Nanoplatelets

Substitutional metal doping is a powerful strategy for manipulating the emission spectra and excited state dynamics of semiconductor nanomaterials. Here, we demonstrate the synthesis of colloidal manganese (Mn²⁺)-doped organic-inorganic hybrid perovskite nanoplatelets (chemical formula: L₂[APb_1-xMn_xBr₃]_n-1Pb_1-xMn_xBr₄; L, butylammonium; A, methylammonium or formamidinium; n (= 1 or 2), number of Pb_1-xMn_xBr₆^4- octahedral layers in thickness) via a ligand-assisted reprecipitation method. Substitutional doping of manganese for lead introduces bright (approaching 100% efficiency) and long-lived (>500 μs) midgap Mn²⁺ atomic states, and the doped nanoplatelets exhibit dual emission from both the band edge and the dopant state. Photoluminescence quantum yields and band-edge-to-Mn intensity ratios exhibit strong excitation power dependence, even at a very low incident intensity (<100 μW/cm²). Surprisingly, we find that the saturation of long-lived Mn²⁺ dopant sites cannot explain our observation. Instead, we propose an alternative mechanism involving the cross-relaxation of long-lived Mn-site excitations by freely diffusing band-edge excitons. We formulate a kinetic model based on this cross-relaxation mechanism that quantitatively reproduces all of the experimental observations and validate the model using time-resolved absorption and emission spectroscopy. Finally, we extract a concentration-normalized microscopic rate constant for band edge-to-dopant excitation transfer that is ∼10× faster in methylammonium-containing nanoplatelets than in formamidinium-containing nanoplatelets. This work provides fundamental insight into the interaction of mobile band edge excitons with localized dopant sites in 2D semiconductors and expands the toolbox for manipulating light emission in perovskite nanomaterials.

The effect of metal substitution in CsSnI3 perovskites with enhanced optoelectronic and photovoltaic properties

Non-toxic lead-free halide metal perovskites have gained significant interest in photovoltaic and optoelectronic device applications. In this manuscript, we have studied the structural, electronic, mechanical, and optical properties of eco-friendly cubic CsSn_1-x Cu _x I₃, (x = 0, 0.125, 0.25, 0.5, 1) perovskites applying first-principles pseudopotential-based density functional theory (DFT). Cu-doped CsSnI₃ has a large impact on the band gap energy viz. the transition of direct band gap towards the indirect band gap. The mechanical properties demonstrate that the pristine and Cu-doped CsSnI₃ samples are mechanically stable and their ductility is enhanced by Cu doping. The mechanical stability and ductility favors the suitability of pure and Cu-doped samples in the thin film industry. The absorption edge of Cu-doped CsSnI₃ moves towards the lower energy region in comparison with their pure form. In addition, the high dielectric constant, high optical absorption, and high optical conductivity of Cu-doped CsSnI₃ materials suggests that the studied materials have a broad range of applications in optoelectronic devices, especially solar cells. A combined analysis of the structural, electronic, mechanical and optical properties suggests that CsSn_1-x Cu _x I₃, (x = 0, 0.125, 0.25, 0.5, 1) samples are a suitable candidate for photovoltaic as well as optoelectronic device applications.

Efficient light-emitting diodes based on oriented perovskite nanoplatelets

Solution-processed planar perovskite light-emitting diodes (LEDs) promise high-performance and cost-effective electroluminescent devices ideal for large-area display and lighting applications. Exploiting emission layers with high ratios of horizontal transition dipole moments (TDMs) is expected to boost the photon outcoupling of planar LEDs. However, LEDs based on anisotropic perovskite nanoemitters remain to be inefficient (external quantum efficiency, EQE <5%) due to the difficulties of simultaneously controlling the orientations of TDMs, achieving high photoluminescence quantum yields (PLQYs) and realizing charge balance in the films of assembled nanostructures. Here, we demonstrate efficient electroluminescence from an in situ grown perovskite film composed of a monolayer of face-on oriented nanoplatelets. The ratio of horizontal TDMs of the perovskite nanoplatelet film is ~84%, which leads to a light-outcoupling efficiency of ~31%, substantially higher than that of isotropic emitters (~23%). In consequence, LEDs with a peak EQE of 23.6% are achieved, representing highly efficient planar perovskite LEDs.

State of the Art and Prospects for Halide Perovskite Nanocrystals

Metal-halide perovskites have rapidly emerged as one of the most promising materials of the 21st century, with many exciting properties and great potential for a broad range of applications, from photovoltaics to optoelectronics and photocatalysis. The ease with which metal-halide perovskites can be synthesized in the form of brightly luminescent colloidal nanocrystals, as well as their tunable and intriguing optical and electronic properties, has attracted researchers from different disciplines of science and technology. In the last few years, there has been a significant progress in the shape-controlled synthesis of perovskite nanocrystals and understanding of their properties and applications. In this comprehensive review, researchers having expertise in different fields (chemistry, physics, and device engineering) of metal-halide perovskite nanocrystals have joined together to provide a state of the art overview and future prospects of metal-halide perovskite nanocrystal research.

Toward Engineering Intrinsic Line Widths and Line Broadening in Perovskite Nanoplatelets

Perovskite nanoplatelets possess extremely narrow absorption and emission line widths, which are crucial characteristics for many optical applications. However, their underlying intrinsic and extrinsic line-broadening mechanisms are poorly understood. Here, we apply multidimensional coherent spectroscopy to determine the homogeneous line broadening of colloidal perovskite nanoplatelet ensembles. We demonstrate a dependence of not only their intrinsic line widths but also of various broadening mechanisms on platelet geometry. We find that decreasing nanoplatelet thickness by a single monolayer results in a 2-fold reduction of the inhomogeneous line width and a 3-fold reduction of the intrinsic homogeneous line width to the sub-millielectronvolts regime. In addition, our measurements suggest homogeneously broadened exciton resonances in two-layer (but not necessarily three-layer) nanoplatelets at room-temperature.

Efficient interlayer exciton transport in two-dimensional metal-halide perovskites

Two-dimensional (2D) metal-halide perovskites are attractive for use in light harvesting and light emitting devices, presenting improved stability as compared to the more conventional three-dimensional perovskite phases. Significant attention has been paid to influencing the layer orientation of 2D perovskite phases, with the charge-carrier transport through the plane of the material being orders of magnitude more efficient than the interlayer transport. Importantly though, the thinnest members of the 2D perovskite family exhibit strong exciton binding energies, suggesting that interlayer energy transport mediated by dipole-dipole coupling may be relevant. We present transient microscopy measurements of the interlayer energy transport in the (PEA)₂PbI₄ perovskite. We find efficient interlayer exciton transport (0.06 cm² s^-1), which translates into a diffusion length that exceeds 100 nm and a sub-ps timescale for energy transfer. While still slower than the in-plane exciton transport (0.2 cm² s^-1), our results show that excitonic energy transport is considerably less anisotropic than charge-carrier transport for 2D perovskites.

Nanoscale Mapping of Morphology of Organic Thin Films

We determine precise nanoscale information about the morphologies of several organic thin film structures using Fourier plane imaging microscopy (FIM). We used FIM microscopy to detect the orientation of molecular transition dipole moments from an extremely low density of luminescent dye molecules, which we call "morphology sensors". The orientation of the sensor molecules is driven by the local film structure and thus can be used to determine details of the host morphology without influencing it. We use symmetric planar phosphorescent dye molecules as the sensors that are deposited into the bulk of organic film hosts during the growth. We demonstrate morphological mapping with a depth resolution to a few Ångstroms that is limited by the ability to determine thickness during deposition, along with an in-plane resolution limited by optical diffraction. Furthermore, we monitor morphological changes arising from thermal annealing of metastable organic films that are commonly employed in photonic devices.

Phase Control and In Situ Passivation of Quasi-2D Metal Halide Perovskites for Spectrally Stable Blue Light-Emitting Diodes

The fabrication of efficient and spectrally stable pure-blue perovskite light-emitting diodes (LEDs) has been elusive and remains of great interest. Herein, we incorporate diammonium salts into quasi-2D perovskite precursors for phase control of multiple quantum well structures to yield tunable and efficient emission in the blue region. With detailed characterizations and computational studies, we show that in situ passivation by the diammonium salts effectively modifies the surface energies of quasi-2D phases and inhibits the growth of low-band gap quasi-2D and 3D phases. Such phase control and in situ passivation could afford blue light-emitting perovskite thin films with high photoluminescence quantum efficiencies of, for instance, 75% for the emission peak at 471 nm. Using this perovskite thin film as an emitting layer, spectrally stable pure-blue LEDs with an emission peak at 474 nm and a full width at half-maximum of 26 nm could be fabricated to exhibit a brightness of 290 cd m^-2 at 8 V and an external quantum efficiency of 2.17%.

Effect of emitter orientation on the outcoupling efficiency of perovskite light-emitting diodes

Metal halide perovskite light-emitting diodes (PeLEDs) have experienced a rapid advancement in the last several years with the external quantum efficiencies (EQEs) reaching over 20%, comparable to the state-of-the-art organic LEDs and quantum dot LEDs. The photoluminescence quantum yields of perovskite films have also been approaching 100%. Therefore, the next step to improving the EQE of PeLEDs should be focused on boosting light extraction. In this Letter, we demonstrate the emitter dipole orientation as a key parameter in determining the outcoupling efficiency of PeLEDs. We find that the CsPbBr₃ emitter has a slightly preferred orientation with the horizontal-to-vertical dipole ratio of 0.41:0.59, as compared to 0.33:0.67 in the isotropic case. A theoretical analysis predicts that a purely anisotropic perovskite emitter may result in a maximum EQE of 36%.

Photonic-Crystal Scintillators: Molding the Flow of Light to Enhance X-Ray and γ-Ray Detection

Scintillators are central for detection of γ-ray, x-ray, and high energy particles in various applications, all seeking higher scintillation yield and rate. However, these are limited by the intrinsic isotropy of spontaneous emission of the scintillation light and its inefficient outcoupling. We propose a new design methodology for scintillators that exploits the Purcell effect to enhance their light emission. As examples, we show 1D photonic crystals from scintillator materials that achieve directional emission and fivefold enhancement in the number of detectable photons per excitation.

Printing and In Situ Assembly of CdSe/CdS Nanoplatelets as Uniform Films with Unity In-Plane Transition Dipole Moment

Distribution of the transition dipole moments (TDMs) of light emitters can intrinsically affect the light out-coupling efficiency of planar light-emitting diodes (LEDs). Lacking the control of TDM distribution has limited the efficiency of nanocrystal-based LEDs to 20%. Here, we present a method that deposits uniform nanocrystal films with unity in-plane TDM distribution. Combining an inkjet printing technique and colloidal nanocrystal self-assembly, we achieved direct printing and in situ assembly of colloidal CdSe/CdS nanoplatelets to all orient "face-down" on various substrates. With motorized translation stages, pattern printing is realized, which demonstrates the potential for integration in industrial-scale fabrication. The method is applied to achieve uniform nanoplatelet films with unity in-plane TDM distribution on zinc-oxide films, a commonly used electron-transport layer. Thus, our work paves the way to break the light out-coupling efficiency limitation of 20% in state-of-the-art nanocrystal-based LEDs, which exclusively possess an isotropic TDM distribution.

Exploring the Origin of Phase-Transformation Kinetics of CsPbI3 Perovskite Nanocrystals Based on Activation Energy Measurements

Perovskite α-CsPbI₃ nanocrystals (NCs) with a high fluorescence quantum yield (QY) typically undergo a rapid phase transformation to a low-QY δ-CsPbI₃ phase, thus limiting their optoelectronic applications. In this study, organic molecule hexamethyldisilathiane (HMS) is used as a unique surfactant to greatly enhance the stability of the cubic phase of CsPbI₃ NCs (HMS-CsPbI₃) under ambient conditions. The reaction kinetics of the phase transformation of CsPbI₃ NCs are systemically investigated through in situ photoluminescence (PL), X-ray diffraction, and transmission electron microscope (TEM) measurements under moisture. The activation energy of HMS-CsPbI₃ NCs is found to be 14 times larger than that of CsPbI₃ NCs capped by olyelamine (OLA-CsPbI₃ NCs). According to density functional theory calculations, the bonding between HMS and CsPbI₃ NCs is stronger than that between OLA and CsPbI₃ NCs, preventing the subsequent phase transformation. Our study presents a clear pathway for achieving highly stable CsPbI₃ NCs for future applications.

Directional Light Emission from Layered Metal Halide Perovskite Crystals

Metal halide perovskites are being increasingly explored for use in light-emitting diodes (LEDs), with achievements in efficiency and brightness charted across the spectrum. One path to further boosting the fraction of useful photons generated by injected electrical charges will be to tailor the emission patterns of devices. Here we investigate directional emission from layered metal halide perovskites. We quantify the proportion of in-plane versus out-of-plane transition dipole components for a suite of layered perovskites. We find that certain perovskite single crystals have highly anisotropic emissions and up to 90% of their transition dipole in-plane. For thin films, emission anisotropy increases as the nominal layer thickness decreases and is generally greater with butylammonium cations than with phenethylammonium cations. Numerical simulations reveal that anisotropic emission from layered perovskites in thin-film LEDs may lead to external quantum efficiencies of 45%, an absolute gain of 13% over equivalent films with isotropic emitters.

Scalable photonic sources using two-dimensional lead halide perovskite superlattices

Miniaturized photonic sources based on semiconducting two-dimensional (2D) materials offer new technological opportunities beyond the modern III-V platforms. For example, the quantum-confined 2D electronic structure aligns the exciton transition dipole moment parallel to the surface plane, thereby outcoupling more light to air which gives rise to high-efficiency quantum optics and electroluminescent devices. It requires scalable materials and processes to create the decoupled multi-quantum-well superlattices, in which individual 2D material layers are isolated by atomically thin quantum barriers. Here, we report decoupled multi-quantum-well superlattices comprised of the colloidal quantum wells of lead halide perovskites, with unprecedentedly ultrathin quantum barriers that screen interlayer interactions within the range of 6.5 Å. Crystallographic and 2D k-space spectroscopic analysis reveals that the transition dipole moment orientation of bright excitons in the superlattices is predominantly in-plane and independent of stacking layer and quantum barrier thickness, confirming interlayer decoupling.

Decoupled high-order MQW superlattices are desired to design miniaturized photonic sources but they are yet to be realized in scalable ways. Here Jagielski et al. achieve this goal using multiple-stacked colloidal lead halide perovskite quantum wells separated by atomically thin quantum barriers.

Effect of dimensionality on the optical absorption properties of CsPbI3 perovskite nanocrystals

The bandgaps of CsPbI₃ perovskite nanocrystals are measured by absorption spectroscopy at cryogenic temperatures. Anomalous bandgap shifts are observed in CsPbI₃ nanocubes and nanoplatelets, which are modeled accurately by bandgap renormalization due to lattice vibrational modes. We find that decreasing dimensionality of the CsPbI₃ lattice in nanoplatelets greatly reduces electron-phonon coupling, and dominant out-of-plane quantum confinement results in a homogeneously broadened absorption line shape down to cryogenic temperatures. An absorption tail forms at low-temperatures in CsPbI₃ nanocubes, which we attribute to shallow defect states positioned near the valence band edge.

Low-dimensional perovskite nanoplatelet synthesis using in situ photophysical monitoring to establish controlled growth

Perovskite nanoparticles have attracted the attention of research groups around the world for their impressive photophysical properties, facile synthesis and versatile surface chemistry. Here, we report a synthetic route that takes advantage of a suite of soluble precursors to generate CsPbBr₃ perovskite nanoplatelets with fine control over size, thickness and optical properties. We demonstrate near unit cell precision, creating well characterized materials with sharp, narrow emission lines at 430, 460 and 490 nm corresponding to nanoplatelets that are 2, 4, and 6 unit cells thick, respectively. Nanoplatelets were characterized with optical spectroscopy, atomic force microscopy, scanning electron microscopy and transmission electron microscopy to explicitly correlate growth conditions, thickness and resulting photophysical properties. Detailed in situ photoluminescence spectroscopic studies were carried out to understand and optimize particle growth by correlating light emission with nanoplatelet growth across a range of synthetic conditions. It was found that nanoplatelet thickness and emission wavelength increase as the ratio of oleic acid to oleyl amine or the reaction temperature is increased. Using this information, we control the lateral size, width and corresponding emission wavelength of the desired nanoplatelets by modulating the temperature and ratios of the ligand.

Device Engineering for All-Inorganic Perovskite Light-Emitting Diodes

Recently, all-inorganic perovskite light-emitting diodes (PeLEDs) have attracted both academic and industrial interest thanks to their outstanding properties, such as high efficiency, bright luminance, excellent color purity, low cost and potentially good operational stability. Apart from the design and treatment of all-inorganic emitters, the device engineering is another significant factor to guarantee the high performance. In this review, we have summarized the state-of-the-art concepts for device engineering in all-inorganic PeLEDs, where the charge injection, transport, balance and leakage play a critical role in the performance. First, we have described the fundamental concepts of all-inorganic PeLEDs. Then, we have introduced the enhancement of device engineering in all-inorganic PeLEDs. Particularly, we have comprehensively highlighted the emergence of all-inorganic PeLEDs, strategies to improve the hole injection, approaches to enhance the electron injection, schemes to increase the charge balance and methods to decrease the charge leakage. Finally, we have clarified the issues and ways to further enhance the performance of all-inorganic PeLEDs.